Stroke is a highly prevalent and devastating disease with limited therapeutic options. Most efforts to develop treatments for stroke have thus far met with limited success in large clinical trials. The brain has an enormous capacity for self-preservation, and elucidating how the brain protects itself may provide novel insights into stroke treatment. Ischemic tolerance (IT), a phenomenon in which application of sub-lethal stress [preconditioning (PC) stimulus; PC] induces a state of tolerance to a subsequent ischemic insult, represents an example of endogenous neuroprotection. IT can be induced in brain by systemic administration of the Toll-like receptor-4 ligand lipopolysaccharide (LPS). Different populations of immune cell could potentially play a role in PC induced by LPS. In particular, monocytes, which are the main target of LPS, can exert a powerful protective effect either directly or by modulating the immune system. However, it remains to be established whether monocytes or other immune cells play a role in the IT induced by LPS. The identification of the cell type(s) responsible for LPS induced IT would be important as it would offer the prospect of novel therapies for ischemic stroke based on these protective immune cells. This application will test the hypothesis that LPS PC induces brain engraftment of protective monocytic cells that ameliorate ischemic brain injury either directly or by modulating the immune response to cerebral ischemia. To this end, we will determine (a) the origin of neuroprotective monocytic subtypes, (b) the molecular factors driving their entry into the brain, and (c) the mechanisms by which they confer neuroprotection. Furthermore, we will determine whether these cells can be adoptively transferred to protect animals from experimental stroke with a clinically relevant therapeutic window. These goals will be achieved using a mouse model of transient focal cerebral ischemia with assessment of histological and neurological outcome. The effect of LPS PC on immune cell populations will be assessed by flow cytometry. PUBLIC HEALTH RELEVANCE: Stroke caused by insufficient blood flow to the brain (ischemic stroke) is a leading cause of brain damage and death, but there is a paucity of therapies to protect the brain from its devastating effects. Our studies will explore new ways to dampen stroke damage based on the body's own ability to protect the brain. In particular, we will examine whether certain immune cells (monocytes) can make the brain more tolerant to the lack of blood that causes the stroke. The findings offer the prospect of using these monocytes as a novel treatment for acute ischemic stroke.